1. Field of the Invention
The invention relates to a device and a method for comminuting coarsely crushed polycrystalline silicon of such a purity that it can be used directly, i.e. without subsequent cleaning, for photovoltaic applications.
2. Description of the Related Art
Polycrystalline silicon (polysilicon) is usually produced by gas vapor deposition in a Siemens reactor. This involves depositing high-purity silane or chlorosilane on a hot substrate (preferably silicon), so that solid rods, ingots or slabs are obtained. Before this polysilicon can be used in crystallization processes, it must be comminuted. This usually causes it to be contaminated by abraded matter, with the result that the contaminants on the surface have to be removed by cleaning methods.
Polysilicon that can be used directly in the crystallization processes (i.e. without subsequent cleaning) and is suitable for solar applications, i.e. is very pure (total metallic contamination typically <10 ppba) could until now only be obtained by laborious, personnel-intensive manual comminution. In a first step, a silicon rod, as obtained from a Siemens depositing reactor, was pre-crushed with a hand hammer and subsequently post-comminuted to the required size of fragments by hand with a riveting hammer. This manually produced coarse crushed material could be further comminuted mechanically by feeding it to a machine.
Previously described mechanical crushing methods that are suitable for comminuting coarsely crushed polycrystalline silicon, for example customary jaw crushers or roll crushers, either cause excessive metallic surface contamination (conventional jaw crushers about 500 to 1000 parts per billion atoms (ppba), conventional roll crushers about 200 to 500 ppba), which requires laborious subsequent cleaning, or are uneconomical due to very complex machine configurations or laborious methods (for example shockwave comminution or thermal crushing).
When roll crushers are used, the suitable size of the feedstock depends on the angle of nip, and consequently on the dimensioning of the crushing machine. On account of the geometry of the rolls, it has so far only been possible to feed in fragments with a maximum edge length of <110 mm. In addition, a number of crushing cycles have been necessary to produce fine-particles products. For engineering reasons, roll crushers with hard-metal crushing tools have so far only been used with a roll diameter of ≦450 mm. Furthermore, roll maintenance is an additional cost factor. On account of the technical configuration of the hard-metal rolls, exchange or replacement of the rolls is very time-consuming and cost-intensive.